This is probably preaching to the converted on this forum, but thought I'd share my recent Laptop HD replacement experience.
My 2yr old Toshiba was showing it's age of late, with a small (fullish) drive and those new widescreen notebooks looking sharp, fast and cheap, I figured my old Toshiba's days were numbered.
Knowing full well that the performance would be better with a new drive, I didn't realize just how large an improvement it would be. After a 10-15min installation and data xfer, my old Toshiba satellite can run with the new laptops just fine, IMO.
What a difference. My laptop is now also much quieter (got a $100 Samsung replacement), and looks like it will serve me for another few years (I mainly use my laptop for letters, email, internet, etc. and leave the big jobs and development to my desktop, which still blows away even the quickest laptops.)
So if you are thinking of retiring your old laptop, try a new drive in it instead and the results may surprise you.